Propellent grain



April 10, 1962 w. PROELL PROPELLENT GRAIN Filed May 17, 1952 R H m m M mmA m n W W Y B United States Patent 3,028,810 PROPELLENT GRAIN Wayne A.Proell, Chicago, Ill., assignor to Standard Oil Company, Chicago, 111.,a corporation of Indiana Filed May 17, 1952, Ser. No. 288,550 12 Claims.(Cl. 10298) This invention relates to an article of manufacture suitablefor the generation of gases at high pressure by the decomposition ofammonium nitrate. More particularly, the invention relates to anexplosive grain consisting of a body portion and an outer surfacecoating whereby the ignition of the grain at low pressure isfacilitated.

Ammonium nitrate is classified as a high explosive even though it isextremely insensitive and cannot be readily ignited by the localapplication of heat or by a blasting cap. Even when ignited, ammoniumnitrate alone does not consistently sustain the propagation ofcombustion. In order to utilize the excess oxygen available from thedecomposition of ammonium nitrate, oxidizable materials are mixed withthe ammonium nitrate. However, while these mixtures of ammonium nitrateand oxidizable material are more sensitive than the ammonium nitratealone, the mixtures are also insensitive or slow burning.

It is known that a smooth burning, readily ignitable ammoniumnitrate-oxidizable material mixture can be obtained by the use ofchromium-type combustion catalyst. The more common chromium-typecombustion catalysts are ammonium or alkali metal chromates orpolychromates, chromic oxide, chromic nitrate and copper chromite. Thepreferred combustion catalyst for commercial mixtures is ammoniumdichromate. The relative toxicity of the chromates makes them hazardousunless handled with considerable care. A particular disadvantage to theuse of the chromate catalyst is their strong oxidizing property; thechromates react slowly with the oxidizable material in the mixture togive chromate salts which are relatively ineffective catalyticallythuson long storage the chromate containing mixture loses its sensitivity toignition.

It has been discovered that certain iron compounds are effectivecatalysts for the combustion of ammonium nitrate and ammoniumnitrate-oxidizable material mixtures. These catalysts are the subjectmatter of US. patent applications filed by Wayne A. Proell and WilliamG. Stanley, Serial Number 273,564, filed February 26, 1952, now PatentNo. 2,987,389, and Serial Number 288,065, filed May 15, 1952, now PatentNo. 2,955,033. All of the combustion catalysts disclosed in theseapplications contain the iron cyanide radical, either ferrocyanide orferricyanide. In addition to the iron cyanide radical, these catalystscontain a second iron ion which may be either ferric or ferro(us). Inaddition to the iron-iron cyanide complex, the catalyst may containalkali metal and/ or ammonium ions. It has been found that the genericclasses of iron-iron cyanide compounds known as soluble Prussian bluesand insoluble Prussian blues are effective catalysts for the purposes ofthis invention. It is known that the better soluble Prussian bluescontain alkali metal(s) such as potassium and sodium and/or the ammoniumradical. Some of the compounds which have been found to be effectiveare: ferro ferrocyanide, ferric ferrocyanide, ferro ferricyanide, ferricferricyanide, potassium ferric ferrocyanide, sodium ferric ferrocyanide,ammonium ferric ferrocyanide, potassium soluble Prussian blue, sodiumsoluble Prussian blue and ammonium-sodium soluble Prussian blue.

The above applications disclose that insoluble Prussian blue is the moreeffective catalyst at elevated pressures such as exist in the combustionchamber of a rocket motor, e.g., 500 to 2000 psi. However, the insolublePrussian blue containing ammonium nitrate-oxidizable 3,028,810 PatentedApr. 10, 1962 ice material mixture is very diflicult to ignite at lowpressure, e.g., about 15 p.s.i. when the catalyst content is below about6 weight percent. The soluble Prussian blue containing mixture willignite readily at low pressure when using as little as 3 to 4% of thecatalyst, but the combustion is not self-sustaining in an inertatmosphere.

It has been discovered that ammoniated insoluble Prussian blue is aneffective catalyst for the combustion of ammonium nitrate or ammoniumnitrate-oxidizable material mixtures. The ammoniated insoluble Prussianblue catalyst possesses the ignition characteristics of the solublePrussian blue catalyst and the burning rate characteristics of theinsoluble Prussian blue catalyst. When used in the mixture in amounts ofabout 3 to 4%, the ammoniated insoluble Prussian blue catalyzed mixtureis hard to ignite and does not sustain combustion in an inertatmosphere. The ammoniated insoluble Prussian blue catalyst is producedby exposing an insoluble Prussian blue to the action of ammonia gas. Thetemperature of the reaction zone containing the insoluble Prussian blueand ammonia gas increases rapidly until a temperature of about 60 C. isreached; as the temperature increases, the rate of increase decreasesuntil at about 60 C. a plateau is reached. As measured by temperatureincrease, the interaction of ammonia and the insoluble Prussian blue isbelieved to substantially stop when the temperature of the reaction zonereaches the plateau of about 60 C. The ammoniated insoluble Prussianblue has a strong odor of ammonia after being cooled to roomtemperature. An ammoniated insoluble Prussian blue of catalytic activityabout equal to the odorous material which does not possess anyappreciable ammonia odor can be obtained by maintaining the odorousmaterial at a temperature of about 70 C. for several hours. Theammoniation of the insoluble Prussian blue does not change the physicalappearance of the material and is noticeable principally in that thecatalytic activity of the insoluble Prussian blue, particularly at lowoperating pressures, is markedly improved. By this treatment variouscatalytically active grades of insoluble Prussian blue can be convertedto materials having about equal catalytic activity, i.e., the normalvariation in catalytic activity of Prussian blue obtained from differentmanufacturers can be eliminated by this ammoniation procedure. The termammoniated insoluble Prussian blue is intended to include both theammonia-odorous material and the substantially odorfree material.

The above described iron cyanide containing catalysts have a verydesirable characteristic in that the burning rate increases onlyslightly with increase in catalyst content of the mixture. The burningrate is defined as the speed, inches per second, with which acylindrical grain about /4 inch in diameter and about 6 inches longburns. The burning rate increases with increase in pressure at which theburning takes place. The burning rate at elevated pressures is normallydetermined by the use of a so-called Crawford bomb. When using about 2weight percent of insoluble Pressian blue as the catalyst, and about 20weight percent of oxygenated oxidizable material, and the remainderessentially ammonium nitrate, a burning rate at 1000 p.s.i. of about0.12 in./sec. is obtained. At a catalyst composition of about 10 Weightpercent, and a corresponding decrease in ammonium nitrate and oxygenatedoxidizable material, a burning rate of about 0.15 in./sec. is obtained.

It is preferred for the purposes of gas generation at high pressures touse a minimum amount of combustion catalyst such as 0.5 to about 4weight percent, preferably 1 to 3 weight percent. At these low catalystcontents the mixture is virtually non-ignitable at ordinary pressures. Ashaped grain having this preferred low catalyst content can be made tooperate by pressuring the combustion chamber to an elevated pressureprior to the application of the ignition means to the surface of thegrain. A common procedure for this pressuring is to have a charge ofblack powder present in the combustion chamber; the burning of the blackpowder raises the pressure in the chamber to in excess of 500 p.s.i. andalso ignites the explosive grain. The use of this pressuring means isundesirable because 1) it requires the presence of an additional item inthe chamber and (2) normally there exists in the chamber prior to theignition of the explosive grain a pressure surge in excess of thedesired operating pressure of the gas generating apparatus.

An object of this invention is an ammonium nitrateoxidizable materialexplosive grain that is readily ignitable at ordinary pressures. Anotherobject is the manufacture of an explosive grain which is readilyignitable at ordinary pressures and does not require a high combustioncatalyst content. Still another object is the manufacture of a compositeexplosive grain consisting of a body portion having a desirably lowcatalyst content and an outer surface coating which permits easyignition at low pressures and which coating has a burning rate aboutthat of the body portion.

The above objects and other objects which will be apparent in thedetailed description of the invention are achieved as follows: The novelexplosive grain of this invention consists of a body portion and anouter surface coating portion, which coating has a thickness betweenabout 1 and 20 millimeters. The body portion comprises essentially amixture of ammonium nitrate, about 71 to 95 weight percent; oxidizablematerial, about to 25 weight percent; combustion catalyst, between about0.5 and 4 weight percent. The outer surface coating comprisesessentially ammonium nitrate, about 63 to 89 weight percent; oxidizablematerial, between about 5 and 25 weight percent; and combustioncatalyst, between about 6 and 12 weight percent. The combustion catalystused in the composite explosive grain of this invention is selected fromthe group consisting of iron-iron cyanide complexes, soluble Prussianblues, insoluble Prussian blues and ammoniated insoluble Prussian blues.

While the amount of combustion catalyst needed to obtain easy ignitionat ordinary pressures of the surface coating will vary somewhatdependent on the particular catalyst and a particular oxidizablematerial, in general about 6% is sufficient. With increase in catalystcontent the ease of ignition improves, but no real advantage is gainedby using more than about 12% The surface coating performs two functionswhich are 1) permits easy ignition at ordinary temperatures, i.e., aboutatmospheric pressure, and (2) produces suificient gas to pressure thecombustion chamber to a pressure such that the body portion will ignite.Thus the minimum thickness of the outer surface coating will vary withthe composition of the body portion and also with the free volume of thecombustion chamber, i.e., the larger the free volume, the more gasneeded to bring the combustion chamber to the desired elevated pressure.In general the thickness of the outer surface coating may be from about1 mm. to about 20 mm. or more. Preferably the thickness of the outersurface coating should be between about 2 and 10 mm. Further, it may bedesirable to increase the thickness of the outer surface coating whenoperating with very large diameter grains such as 8 inches or 12 inches.

Normally in the generation of gas by the burning of an ammonium nitrateexplosive, only a part of the total surface is permitted to burn. Thisresult is achieved by coating some of the surface with an inhibitingmaterial which prevents the ignition of the ammonium nitrate beneath theinhibiting material. Examples of suitable inhibiting material arecellulose acetate, Scotch cellophane tape and petroleum asphalts. Forexample: A cylindrical grain can be burned cigarette-fashion byinhibiting the cylindrical surface and one end of the grain. On theother hand, a cylindrical grain can be burned along the cylindricalsurface only by inhibiting both ends of the grain. A cylindrical grainwhich is provided with a coaxial longitudinal opening throughout thelength of the grain can be made to burn on both the outer and innercylindrical surfaces by inhibiting the annular area of both ends. Theexplosive grain of this invention is not limited to a perforatedcylinder (tubular) grain, but may be utilized with any shaped grain suchas cruciform, triform, hexaforrn, slab, etc. It is normal practice toinhibit a part of the outer surface of all grains used for propellingrockets. A grain which consists of a body portion and inhibitor placedon part of said body portion and the outer surface coating of thisinvention over the uninhibited surface is within the scope of theinvention. It has been found that all of the burning surface of the bodyportion need not be coated in order to obtain ignition of the entireuninhibited surface of the body portion; however, at least about 70% ofthe uninhibited burning surface should be coated in order to obtaincomplete and uniform ignition.

FIGURE 1 shows a longitudinal cross-sectional view of one embodiment ofthe invention.

FIGURE 2 shows a cross-sectional view along the line 2-2 of thisembodiment.

In the figures the body portion 11 of the propellant grain is providedwith an external coating 12. An internal coating 13 covers thecylindrical surface of a coaxial longitudinal perforation 14. The grainwas forced to burn only on the curved interior and exterior surfaces bythe presence of a layer of inhibitor, 15 and 16 respectively, positionedon the annular end surfaces of the grain. Thus the grain burns both onthe inner surface and the outer surface with the direction of burningproceeding toward the center of the body portion 11.

The term ammonium nitrate as used in this specification and in theclaims is intended to mean either ordinary commercial grade ammoniumnitrate, such as, conventionally grained ammonium nitrate containing asmall amount of impurities and which is then generally coated with asmall amount of moisture-resisting material such as petrolatum orparafiin, or military grade ammonium nitrate, or a mixture of otherinorganic nitrates and ammonium nitrate wherein the ammonium nitrate isthe preponderate nitrate.

The decomposition of ammonium nitrate produces freeoxygen in addition tothe other decomposition products. Additional energy can be obtained byhaving present in the combustion zone an oxidizable material whichcombines with the free-oxygen. It is to be understood that thecombustion catalysts utilized in the explosive grain of this inventioncontain oxidizable materials which will consume some of the free-oxygen.The oxidizable material may be any material which contains a deficiencyof combined oxygen. Metals such as aluminum and magnesium may be used.The non-metallic elements, sulfur and carbon, may be used.Nitrogen-containing organic compounds that do not unduly sensitize theexplosive mixture are particularly good; examples of these are urea,nitroguanidine, mononitrate naphthalene, dinitrodiphenyl oxide, etc.Hydrocarbon materials are an excellent oxidizable material and arepreferred where low cost is a matter of concern. Examples of hydrocarbonoxidizable materials are waxes, tars, asphalts, bitumen, coal tar, shaleoil residue, viscous higher boiling hydrocarbon oils, etc.

Oxygenated materials are very useful as oxidizable materials for certainapplications of the explosive grain. For use in rockets, a material thatis dimensionally stable is desirable. It has been found that the variouscellulose acetates which have been plasticized to improve workabilityare a particularly suitable oxidizable material. It is desirable to usean oxygenated plasticizer and a polyester of a glycol and a dicarboxylicacid is a particularly suitable plasticizer material. The termoxygenated oxidizable material is intended to include all organicmaterials which contain oxygen as well as carbon and hydrogen--otherelements such as nitrogen and sulfur may also be present.

The amount of oxidizable material that is added to the ammoniumnitrate-catalyst mixture is dependent upon the amount of catalystpresent and the particular type of oxidizable material itself. Normallyit is desirable to have present a sufiicient amount of oxidizablematerial so that the explosive mixture is about stoichiometricallybalanced with respect to oxygen content. This preferred composition maybe exceeded, but in general no more oxidizable material should bepresent than can react with the available oxygen to yield a soot-freegas. The presence of some oxidizable material is desirable so that theexplosive mixture of this invention should contain between about and 25%of oxidizable material, and preferably between about 15 and 25% ofoxygenated oxidizable material.

A cylindrical grain of about 4 inches in diameter by 12 inches long andhaving a 1 inch diameter coaxial longitudinal perforation was made ofthe following composition: ammonium nitrate, 74 weight percent;cellulose acetate, 6%; polyester of ethylene glycol and diglycolic acid,9%; dinitrodiphenyl oxide, 9%; insoluble Prussian blue combustioncatalyst, 2%. This grain could not be ignited at atmospheric pressure,but did ignite quite readily at 1000 p.s.i. and gave a burning rate of0.13 in./sec. Another grain of the above dimensions and composition wasprepared. To this grain there was applied a 5 mm. coating over the outerand inner cylindrical parts thereof; the annulus of both ends wasinhibited with a thin film of asphalt. The surface coating consisting of12% of insoluble Prussian blue; ammonium nitrate, 70%; and 18% of theoxygenated oxidizable materials as described for the body portion. Thecoated grain was placed in a miniature rocket motor and the grain wasignited by applying through the nozzle opening the flame from anordinary match to a portion of the coated surface. The combustion spreadrapidly over the entire uninhibited surface and the entire grain wasconsumed.

The outer surface coating may be applied to the body portion in the formof a pasty mixture by means of a putty knife in experimental work, or bymeans of an automatic machinery for commercial production. Also, thecoating could be added to the body portion by placing the body portionin the mold of an injection molding machine and injection molding thecoating onto the burning surface of the body portion. Still anothermethod is to slurry the composition of the surface coating in a solventsuch as acetone and painting the slurry onto the body portion. Manymethods of applying this surface coating will be readily apparent tothose skilled in the art.

I claim:

1. An article of manufacture that is suitable for the generation of gasat elevated pressures, which article consists of a body portion, whichbody comprises essentially ammonium nitrate between about 71 and 95weight percent, an oxidizable material between about 5 and 25 weightpercent, and between about 0.5 and 4 weight percent of a combustioncatalyst selected from the group consisting of iron-iron cyanidecomplexes, soluble Pmssian blues, insoluble Prussian blues, andammoniated insoluble Prussian blues; and an outer surface coatingportion of a thickness between about 1 and 20 millimeters, which coatingcomprises essentially ammonium nitrate between about 63 and 89 weightpercent, an oxidizable material between about 5 and 25 weight percent,and between about 6 and 12 weight percent of a combustion catalystselected from the group consisting of iron-iron cyanide complexes,soluble Prussian blues, insoluble Prussian blues, and ammoniatedinsoluble Prussian blues.

2. The article of claim 1 wherein said coating covers at least about 70%of the burning surface of said body portion.

3. The article of claim 1 wherein at least one combustion catalyst isammoniated insoluble Prussian blue made by treating insoluble Prussianblue with ammonia until the temperature of the reaction zone reachesabout 60 C. and substantially no further temperature rise takes place.

4. The article of claim 1 wherein at least one combustion catalyst isinsoluble Prussian blue.

5. The article of claim 1 wherein at least one combustion catalyst issoluble Prussian blue.

6. The article of claim 1 wherein the catalyst in said body portion isinsoluble Prussian blue and the catalyst in said coating is ammoniatedinsoluble Prussian blue.

7. An article of manufacture that is suitable for the generation of gasat elevated pressures, which article consists of a body portion, whichbody comprises essentially between about 1 and 3 weight percent of acombustion catalyst selected from the group consisting of iron-ironcyanide complexes, soluble Prussian blues, insoluble Prussian blues, andammoniated insoluble Prussian blues, between about 15 and 25 weightpercent of an oxygenated oxidizable material, and the remainderessentially ammonium nitrate; and an outer surface coating having athickness between about 2 and 10 millimeters, which coating comprisesessentially between about 6 and 12 weight percent of a combustioncatalyst selected from the group consisting of iron-iron cyanidecomplexes, soluble Prussian blues, insoluble Prussian blues, andammoniated insoluble Prussian blues, between about 15 and 25 weightpercent of an oxygenated oxidizable material, and the remainderessentially ammonium nitrate.

8. The article of claim 7 wherein said oxygenated oxidizable material isa cellulose acetate plasticized with a polyester of a glycol and adicarboxylic acid.

9. The article of claim 8 wherein said oxygenated oxidizable materialalso contains a nitrodiphenyl oxide.

10. A propellant grain for use in a rocket motor, which grain has acylindrical shape and is provided with a coaxial longitudinal openingthroughout the length of the grain, which grain consists of a shapedbody portion comprising essentially between about 0.5 and 4 weightpercent of a combustion catalyst selected from the group consisting ofiron-iron cyanide complexes, soluble Prussian blues, insoluble Prussianblues, and ammoniated in soluble Prussian blues, between about 5 and 25weight percent of an oxidizable material, and the remainder essentiallyammonium nitrate; and a surface coating which covers a predominantpercentage of the burning surface of said body portion, which coating isbetween about 2 and 10 millimeters in thickness and which comprisesessentially between about 6 and 12 weight percent of a combustioncatalyst selected from the group consisting of iron-iron cyanidecomplexes, soluble Prussian blues, insoluble Prussian blues, andammoniated insoluble Prussian blues, between about 5 and 25 weightpercent of an oxidizable material, and the remainder essentiallyammonium nitrate.

11. The grain of claim 10 wherein an amount of the outer area of saidbody portion is inhibited against combustion.

12. The grain of claim 10 wherein the combustion catalyst is ammoniatedinsoluble Prussian blue and the oxidizable material is an oxygenatedoxidizable material.

References Cited in the file of this patent UNITED STATES PATENTS1,021,882 OBrien Apr. 2, 1912 1,071,949 OBrien Sept. 2, 1913 1,890,112Fisher Dec. 6, 1932 FOREIGN PATENTS 14,196 Great Britain 1897

1. AN ARTICLE OF MANUFACTURING THAT IS SUITABLE FOR THE GENERATION OFGAS AT ELEVATED PRESSURES, WHICH ARTICLE CONSISTS OF A BODY PORTION,WHICH BODY COMPRISES ESSENTIALLY AMMONIUM NITRATE BETWEEN ABOUT 71 AND95 WEIGHT PERCENT, AN OXIDIZABLE MATERIAL BETWEEN ABOUT 5 AND 25 WEIGHTPERCENT, AND BETWEEN ABOUT 0.5 AND 4 WEIGHT PERCENT OF A COMBUSTIONCATALYST SELECTED FROM THE GROUP CONSISTING OF IRON-IRON CYANIDECOMPLEXES, SOLUBLE PRUSSIAN BLUES, INSOLUBLE PRUSSIAN BLUES, ANDAMMONIATED INSOLUBLE PRUSSIAN BLUES, AND AN OUTER SURFACE COATINGPORTION OF A THICKNESS BETWEEN ABOUT 1 AND 20 MILLIMETERS, WHICH COATINGCOMPRISES ESSENTIALLY AMMONIUM NITRATE BETWEEN ABOUT 63 AND 89 WEIGHTPERCENT, AN OXIDIZABLE MATERIAL BETWEEN ABOUT 5 AND 25 WEIGHT PERCENT,AND BETWEEN ABOUT 6 AND 12 WEIGHT PERCENT OF A COMBUSTION CATALYSTSELECTED FROM THE GROUP CONSISTING OF IRON-IRON CYANIDE COMPLEXES,SOLUBLE PRUSSIAN BLUES, INSOLUBLE PRUSSIAN BLUES, AND AMMONIATEDINSOLUBLE PRUSSIAN BLUES.